The present invention relates to a light beam scanning apparatus used in a technical field in which images are recorded when scanning operation is conducted by a light beam such as a laser beam.
As one of this type light beam scanning apparatus, there is provided a internal drum scanning system in which scanning operation is carried out in the following manner: A fixed drum (referred to as a cylinder in this specification) is used. A photosensitive material sheet is attached onto the cylindrical surface. A light beam is circumferentially rotated inside the cylinder so that the photosensitive material sheet on the cylindrical surface can be scanned. In this way, image recording is conducted. This technique is disclosed in Japanese Patent Publication Open to Public Inspection No. 158580/1988.
To go into detail, the primary scanning operation is conducted as follows: A photosensitive material sheet is attached onto the cylindrical surface, and a light beam sent from a light source is incident along a central axis of the cylinder. The light beam is deflected by deflection elements provided in the cylinder such as a reflecting mirror and rectangular prism rotated by a motor around the central axis of the cylinder. Due to the foregoing, the light beam is deflected in a perpendicular direction, and converged by a converging lens disposed in an optical path. In this way, the converged light beam conducts a primary scanning operation on the photosensitive material sheet attached onto the cylindrical surface of the cylinder.
In this case, the motor, light beam deflecting elements and converging lens are moved along the central axis of the cylinder so that the subsidiary scanning operation can be conducted. By the action of the above primary and subsidiary scanning operations, an image is recorded (exposed) on the photosensitive material sheet.
However, in the apparatus described in Japanese Patent Publication Open to Public Inspection No. 158580/1988, only one light beam (a single beam) is used. Therefore, when the light beam deflecting element is rotated by revolution, one primary scanning operation is conducted. Accordingly, an improvement in the recording speed is limited. When a plurality of light beams (multi-beam) are used so as to substantially conduct primary scanning operations by a plurality of times when the light beam deflecting element is rotated by one revolution, it is difficult to scan the plurality of beams in parallel on the scanning surface, that is, it is difficult to conduct scanning using a multi-beam.
A converged beam spot conducts scanning while it is being rotated on the recording surface. Accordingly, when a profile of the spot is elliptic, a direction of the ellipse is changed in accordance with a position of the primary scanning, so that an image of high quality can not be provided. In this case, it is preferable that the beam spot is formed into an ellipse in which a diameter in a direction perpendicular to the primary scanning direction is larger.
In order to solve the above problems, U.S. Pat. No. 5,214,528 discloses the following technique: An image rotation optical element such as a trapezoidal prism by which a beam of light is rotated by one revolution when it is rotated by 1/2 revolution, is disposed as an image rotation optical means on an incident side of a light beam deflection means (light beam deflection element). When this image rotation optical element is rotated at a speed of 1/2 of the rotational speed of the light beam deflection element, a beam of light incident on the light beam deflection element is rotated at the same speed as that of the light beam deflection element. In this way, a plurality of beams of light are scanned in parallel on the scanning surface so that each beam can not be rotated.
Japanese Patent Publication Open to Public inspection No. 199021/1992 discloses the following technique: A rectangular prism is used for the image rotation optical element.
Further, U.S. Pat. No. 5,309,274 discloses the following technique: A reflection type element such as a rectangular prism and a cylindrical lens is used for the image rotation optical element, and a beam of light is incident on the image rotation optical element using a beam splitter. Then a reflected beam of light is made to pass through the beam splitter.
However, in the multi-beam scanning in which the image rotation optical means described in U.S. Pat. No. 5,214,528 is used, unless the holding accuracy for holding the image rotation optical means is very high, a position of the primary scanning line recorded in each period (for each revolution of an image) in which the primary scanning is conducted twice is shifted, so that the image quality is deteriorated.
In the multi-beam scanning described in Japanese Patent Publication Open to Public Inspection No. 199021/1992 in which a rectangular prism type image rotation optical element is used, the following problems may be encountered: A beam of light is incident on a portion separate from a rotational axis of the image rotation optical element by a predetermined distance. Accordingly, a beam of light emergent from the image rotation optical element makes a revolution round the rotational axis, wherein a radius of revolution is the aforementioned distance. Therefore, an optical system located after the image rotation optical element requires a large effective diameter.
In the technique disclosed in U.S. Pat. No. 5,309,274, unless the accuracy of the holding mechanism for holding the image rotation optical element is very high, a position of the primary scanning line recorded in each period (for each revolution of an image)in which the primary scanning is conducted twice is shifted, so that the image quality is deteriorated. Further, a beam of light is rotated by the image rotation optical element, so that the direction of polarization is also rotated. Accordingly, in the case where a beam splitter is used, due to the polarization characteristics of the beam splitter, power of a beam of light fluctuates after it has passed through the beam splitter twice.